You have a few options, the first, if you only need either one or two users is;http://nomachine.com
I'm not sure how licensing works on V4, but they still have a free version (albeit for a limited number of users) and if the license works re; your requirements then it's hands-down the best solution.
Next to that you have either raw X or VNC. The latter is the most efficient, but if you have a local network connection, you will find that raw X is a much better / smoother solution, albeit it'll make more use of your network .. (!)
You'll find lots of documentation on the web for raw X, the only 'real' issues you'll have getting it going are security. Typically you'll want something like this;
a. Make sure you uncomment "ForwardX11 yes" and "ForwardX11Trus
ted yes" in /etc/ssh/ssh_config
b. type "xhost +"
c. type "ssh <user>@<remotehost>
d. type (this will be on <remotehost>) "gnome-terminal" (or a.n.other gui application)
You can have the server present a network login, but that gets a little more complex, you have to deal with something called XDMCP which is a little different for every window manager - documentation for this can be found on the LTSP site.